The objective of this proposal is to study the replication of a eukaryotic gene with a known transcriptive function. The genes which code for 18S and 28S ribosomal RNA in the frog, Xenopus laevis were selected as the system of choice for three reasons. 1) They are readily isolatable in neutral CsC12 gradients. 2) They occur as tandem repeated sequences. 3) Their structure and organization have already been extensively studied. Several specific questions will be asked relating to the replication of these genes: 1) Does DNA synthesis begin at a unique site within a repeating unit? 2) If so, where within the repeat is the replication initiation site located? 3) Do adjacent repeating units begin replication synchronously, or can one initiate DNA synthesis while its neighbor remains dormant? The source of DNA for these experiments will be stage 7 Xenopus blastulae. At this stage the S phase lasts about 10 minutes so that DNA isolated from these embryos should be enriched in replicating molecules. The isolation of ribosomal DNA will be accomplished by isopyknic centrifugation in neutral CsCl and analysis of replicating ribosomal DNA molecules will be performed by electron microscopy before and after cleavage with restriction endonucleases.